I've restored many of these Fine India Steel W&Bs but since I never seem to get the pristine blades I never noticed any evidence of a frost line on them.
My client commissioned me to restore this one and add the frost line back in. It's basically an etch.
I cleaned up the blade ( see,, I don't get pretty ones to just hit with some polish and call it done) polished it, added the frost line and built some Black Horn scales with double stacks at the ivory wedge end and a single at the hinge.
I kept the frost line parallel with the top of the spine, started it at the bottom edge of the shaft and terminated it at the bottom of the barbers notch.
I also built the scales to match the sweep of the top edge of the spine and got the blade to sit "just right" so it paralleled the scales and the script "Diamond Edge Razor" just peeks out of the scales.
Interesting piece which leads me to other possibilities.

It's simply an etched line instead of a panel design on the blade. I think it's a throwback to lines on some knives where you can see the evidence of the heat treating process indicated by a discoloration paralleling the edge on some knives and especially some Samauri blades.